1. Field of the Invention
An apparatus consistent with the present invention relates to a magnetic tape transport system for transporting a magnetic tape, which has multiple magnetic heads arranged parallel to each other in contact with the magnetic tape.
2. Description of the Related Art
In recent years, the high density recording design of magnetic recording media has advanced, and some backup media for computers have a recording capacity of several hundreds of gigabytes. For example, a magnetic tape has several hundreds of data tracks along the width, thereby achieving the high density recording design. Such a high density recording design involves excessive narrowing of the data tracks and of the intervals between the adjacent data tracks in a magnetic tape. Thus, in order to allow recording/reproducing devices of a magnetic head to trace such narrow data tracks, servo signals are written on a magnetic tape in advance, and the servo signals are then read by a magnetic head, while the position of the magnetic head relative to the magnetic tape (the position along the width of the magnetic tape) is servo-controlled (refer to Japanese Unexamined Patent Application No. 8-30942 (Paragraph No. 0004)).
The servo signals are written on corresponding non-magnetized servo bands on a magnetic tape by magnetizing the servo bands in one direction by use of a servo write head of a servo writer, to which a record current is supplied.
In a conventional type magnetic tape MT1 as shown in FIG. 7B, as a means of preventing the saturation of a servo signal read device (not shown) of a magnetic tape drive (not shown) upon reproduction, a record pulse current PC1 having positive pulses and zero pulses as shown in FIG. 7A is generated and used to form servo signals SS1. When the zero pulses of the record pulse current PC1 are supplied to a servo write head of a servo writer, the servo bands SB1 are not magnetized. Meanwhile, when the positive pulses are supplied, parts (servo patterns SP1) of the servo bands SB1 are magnetized in one direction. As a result, the servo signals SS1 are written. The servo write head for writing the servo signal SS1 has head gaps in the form of a symmetric trapezoid at predetermined angle with respective to the transport direction of the magnetic tape MT1. With these head gaps, servo patterns SP1a shown in FIG. 7B are formed when positive pulse currents PPa shown in FIG. 7A are fed to the servo write head, and servo patterns SP1b are formed when positive pulse currents PPb are fed.
A magnetic tape drive uses a servo signal read device to sense magnetic flux transitions of the servo signal SS1, based on a variation in an electric resistance of the servo signal read device, and outputs, as a read signal, a differential voltage waveform produced from the magnetic flux transitions being sensed. Accordingly, as the variation in the electric resistance of the servo signal read device is increased, the peak-to-peak value of the read signal is increased, in other words, the SN ratio of the read signal is enhanced. Specifically, if the magnetic flux transitions of the servo signal SS1 are great, or if the servo signal read device has a large sensing area, then the signal RSL generated upon reading of the servo signal SS1 acquires a large peak-to-peak value, as shown in FIG. 7C.
In the future, it is expected that magnetic tapes will develop to have a recording capacity of several tens of terabytes. As such a high density recording design proceeds, the number of data tracks formed on a magnetic tape is increased, the width of data tracks and the interval between adjacent data tracks are further narrowed, and thus, a magnetic tape itself is thinner. This involves decrease in a magnetic quantity sensed upon reading of the servo signal SS1, that is, decrease in the magnetic flux transitions sensed by the servo signal read device. This causes lowering of the peak value of the voltage signal RSS produced upon reading of the servo signal SS1, as shown in FIG. 7D, and the SN ratio of the signal RSS is thus deteriorated. Consequently, a magnetic tape drive fails to read the servo signal SS1 correctly, and to exactly control the position of the magnetic head.
To overcome this disadvantage, a technique disclosed in JP 2003-110396 (non-published) was conceived by this inventor. In this technique, as shown in FIG. 8A, a servo writer uses a DC erase head (not shown) to magnetize the servo bands SB2 in one direction (i.e. in the right direction in the figure) along the long side of the magnetic tape MT (DC magnetization) and, then uses a servo write head (not shown) to write the servo signals SS on the servo bands SB by magnetizing the parts of the servo bands SB in the opposite direction (i.e. in the left direction in the figure). In this figure, the magnetized directions are denoted by small arrows. The level of the voltage signal, which is generated upon reading of the servo signal SS2, is increased at a border where the orientation of the magnetization is changed. On the servo signal SS2, the magnetic orientations are greatly changed from the right to left directions in the figure, and the magnetic orientations are greatly changed from the left to right directions. Therefore, the voltage signal (see FIG. 8B), which is generated when a servo signal read device S of a magnetic head of a magnetic tape drive reads the servo signal SS2, has a large amplitude, thus improving the SN ratio of the voltage signal.
In addition, when the DC erase head magnetizes the servo bands SB2 in one direction along the long side of the magnetic tape MT2 (hereinafter, this process is referred to as “DC erase”), the DC erase head also DC-erases a data band DB2 to which data signal will be written later, as shown in FIG. 8A. In this case, remanent magnetization on the data band DB2 may cause noise upon reproduction of the data signal.
This disadvantage can be overcome by use of, for example, a servo writer 22 equipped with a DC erase head 20 and a servo write head 21 as shown in FIG. 9. FIG. 9 is a plane view depicting the DC erase head 20 and the servo write head 21 of the servo writer 22, as seen from the direction of the magnetic tape MT2 being transported. Referring to FIG. 9, each head gaps 20a of the DC erase head 20 and each head gap 21a of the servo write head 21 are arranged in the longitudinal (transport) direction of the magnetic tape MT2. Thanks to this arrangement, in writing the servo signals SS2 (see FIG. 8) on the corresponding servo bands SB2 of the magnetic tape MT2 being transported in the right direction of FIG. 9, the DC erase head 20 uses the head gaps 20a to subject only the servo bands SB2 to the DC-erase, and the servo write head 21 then uses the head gaps 21a to write the servo signals SS2 on the DC-erased servo bands SB2. This makes it possible to write the servo signals SS2 on the servo bands SB2 without DC-erasing the data band DB2. In this case, the DC erase head 20 and the servo write head 21 need to be arranged as close to each other as possible to the extent where the cross-talk therebetween does not occur. This is because the servo bands SB2 that have undergone the DC-erase must be prevented from shifting from the corresponding head gap 21a of the servo write head 21, when the magnetic tape MT2 is displaced in the lateral direction (upward and downward directions in FIG. 9) during the transport.
However, when the servo signals SS2 (see FIG. 8A) are written on the servo bands SB2 of the magnetic tape MT2 by the DC erase head 20 and the servo write head 21 arranged close to each other, airflow involved by the transport of the magnetic tape MT2 circulates within an air gap 23 between the two heads 20 and 21. This airflow within the air gap 23 then forces the magnetic tape MT2 to move away from the two heads. In this case, the contact pressure between the magnetic tape MT2 and the DC erase head 20 or the servo write head 21 may be decreased, thereby causing the writing error of the servo signals SS2. In addition, the magnetic tape MT2 may be displaced on magnetic tape contact surfaces 20b and 21b of the DC erase head 20 and the servo write head 21, respectively, in the lateral direction, thereby causing the shift of the DC-erased servo bands SB2 from the respective head gaps 21a of the servo write head 21.
The present invention has been conceived, taking the disadvantages above into account. An object of the present invention is to provide a magnetic tape transport system which is equipped with multiple magnetic heads arranged parallel to each other in contact with a magnetic tape being transported, and which has a structure to prevent a decrease in the contact pressure between the magnetic tape and the magnetic heads.